Generic process for preparing a crystalline oxide upon a group IV semiconductor substrate

ABSTRACT

A process for growing a crystalline oxide epitaxially upon the surface of a Group IV semiconductor, as well as a structure constructed by the process, is described. The semiconductor can be germanium or silicon, and the crystalline oxide can generally be represented by the formula (AO) n  (A&#39;BO 3 ) m  in which &#34;n&#34; and &#34;m&#34; are non-negative integer repeats of planes of the alkaline earth oxides or the alkaline earth-containing perovskite oxides. With atomic level control of interfacial thermodynamics in a multicomponent semiconductor/oxide system, a highly perfect interface between a semiconductor and a crystalline oxide can be obtained.

This invention was made with Government support under Contract No.DE-AC05-960R22464 awarded by the U.S. Department of Energy to LockheedMartin Energy Research Corporation, and the Government has certainrights in the invention.

BACKGROUND OF THE INVENTION

This invention relates generally to the preparation of structuresutilizing a semiconductor-based substrate and relates, moreparticularly, to the formation of crystalline oxide thin films upon asubstrate comprised of a Group IV material, such as germanium orsilicon.

In U.S. Pat. No. 5,225,031, we described a process for depositing anoxide epitaxially onto a silicon substrate so that the structures whichresult from the process would be suitable for use in semiconductor andrelated applications. However, such a discussion was limited to thebuild-up of a crystalline oxide on silicon, a Group IV semiconductormaterial.

For use in some semiconductor devices, such as a transistor, having anepitaxial build-up of a crystalline oxide onto a semiconductor-basedsubstrate, a germanium substrate is likely to provide better operatingcharacteristics than those provided by a silicon substrate. For example,the electron hole mobility of germanium (related to the gain coefficientof the material) and which corresponds to the speed at which current canflow through the material is about four times higher in germanium thanin silicon. Along the same lines, the switching speed (again, a functionof electron hole mobility) is about four times faster in germanium thanin silicon. Consequently, a transistor whose substrate is comprised ofgermanium could theoretically be switched about four times faster than atransistor having a silicon substrate. Therefore, it would be desirableto provide a generic process which can be used for constructing acrystalline oxide upon any Group IV semiconductor material, includinggermanium.

Accordingly, it is an object of the present invention to provide aprocess for growing a thin oxide film epitaxially upon a substratecomprised of elements from Group IV of the periodic table, and inparticular, germanium or silicon.

Another object of the present invention is to provide a structureprepared by the process of the invention.

Yet another object of the present invention is to provide a structurewhich is well-suited for use in semiconductor and related applications.

A further object of the present invention is to provide a ferroelectricfield effect transistor which embodies the structure of this invention.

SUMMARY OF THE INVENTION

This invention resides in improvements to a structure and to anassociated process for growing a crystalline oxide epitaxially upon thesurface of a Group IV semiconductor substrate comprised of germanium orsilicon and wherein the process includes the steps of depositing analkaline earth oxide or an alkaline earth-containing perovskite oxideupon the substrate surface in a layer-by-layer build-up and wherein thealkaline earth oxide or alkaline earth-containing perovskite oxideincludes an alkaline earth metal.

The improvement of the process is characterized in that prior to theinitiation of a build-up of a first oxide layer upon the substrate, thesubstrate surface is passivated against the subsequent reaction withoxygen by forming a monolayer of germanide or silicide.

The structure includes a layup of epitaxial crystalline oxide upon thesurface of a Group IV semiconductor substrate comprised of germanium orsilicon, and the improvement to the structure is characterized by amonolayer of a germanide or a silicide interposed between the substratesurface and the crystalline oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a three-component phase diagram.

FIG. 2 is a perspective view of an embodiment of a structure, shownexploded, which has been constructed in accordance with an embodiment ofthe process of this invention.

FIG. 3 is a schematic perspective view of ultra high vacuum equipmentwith which steps of the present invention can be performed.

FIG. 4 is a scanning transmission electron microscope (STEM) image of across section of a crystalline oxide-on-germanium structure showing aone-monolayer germanide interposed between the crystalline oxide (ofBaTiO₃) and the semiconductor, germanium.

FIG. 5 is a schematic cross-sectional view of a fragment of aferroelectric field effect transistor (FFET) utilizing a perovskite thinfilm as a gate dielectric.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

Since the issuance of applicants' U.S. Pat. No. 5,225,031 whichaddresses a build-up of a crystalline oxide upon silicon (to avoidnative oxide formation, e.g. SiO₂), applicants have experimented furtherwith silicon and other elements, such as germanium, of Group IV of theperiodic table and have developed generalizations of a process which canbe used to build epitaxial, as well as commensurate, crystalline oxideson any Group IV substrate whereby the effects of native oxide formationneed not be taken into account. Such a generalization of the process wasnot possible from the disclosure of U.S. Pat. No. 5,225,031 because theexposure of oxygen to other Group IV materials, and notably germanium,at high temperatures does not result in a native oxide formation, e.g.GeO₂ on germanium.

In the paragraphs which follow, an exemplary build up of crystallineoxide on germanium is described. It will be understood, however, thatthe principles of the present invention can be used for growing oxideson the surface of other Group IV semiconductors.

It is desirable to grow crystalline oxides immediately in contact withGroup IV semiconductors; silicon and germanium are examples. Moreover,it is desirable that the crystalline oxide be epitaxial or evenperfectly commensurate with the semiconductor. In our previous U.S. Pat.No. 5,225,031, we have shown that avoidance of the amorphous nativeoxide, SiO₂, is required if the crystalline silicon template is to beused for establishing the epitaxial interface.

We have discovered that it is now possible to describe a common processthat is applicable to the Group IV semiconductors as a class. There isshown in FIG. 1 the three component phase diagram that is helpful inunderstanding this process for germanium. The three components are puregermanium (Ge), oxygen (O), and an alkaline earth metal (A). Thealkaline earth metal may be any of Ba, Sr, Ca or Mg, but Ba is describedas being used with germanium in the exemplary process which follows.

Stable tie-lines between the components are characteristic of thethermodynamic system, e.g. the line at the base of the triangle in FIG.1 connecting Ge and A. For reaction between Ge and A, we note thatcompounds form as AGe_(x) (wherein the variable "x" can be 0.5, 1, or 2,as examples). In attempting to grow a thin-film of AO against Gefollowing the path indicated by the arrows on the diagram, the interfacebetween Ge and the thin-film AO can be maintained in local thermodynamicequilibrium by first forming AGe_(x) with no oxygen in the system, andthen changing direction in the phase space described in the diagram andgoing toward the AO compound. To make this change in thermodynamicdirection, however, requires that the AGe_(x) compound be exposed to Aand O in appropriate proportions to remain on the stable tie lineextending between AGe_(x) and AO. This concept is the generalizationthat we can now make in the instant application and which has not beenrecognized heretofore: layer-by-layer thermodynamic equilibrium can bemaintained at the atomic level and thereby form stable, heteroepitaxialtransitions from semiconductors to crystalline oxides.

In this connection, a guiding principle of the present invention residesin controlling thermodynamic stability at the atomic level of amulticomponent system during initial stages of interface formationbetween semiconductors and crystalline oxides. If the semiconductor ofinterest is silicon or germanium, the formation of a monolayer ofsilicide or germanide during these initial stages of growth controls theinterface electrostatics, interface strain and interface chemicalcompatibility.

With reference to FIG. 1, there is shown a structure, generallyindicated 18, which has been constructed in accordance with anembodiment of the process of the present invention. The structure 18includes a substrate 20 of pure germanium (a Group IV element), anoverlayer 23 of germanide, a fraction, or fragment 25, of a monolayer ofan alkaline earth metal, an overlayer 29 of alkaline earth oxide, andthen an overlayer 31 of perovskite oxide. In the depicted structure 20,the alkaline earth metal of the monolayer fragment 25 is barium (Ba) andthe perovskite oxide overlayer 31 (which includes the alkaline earthoxide metal barium) is BaTiO₃. As will be apparent herein, steps aretaken to passivate the surface, indicated 22, of the substrate 20 byexposing the surface 22 to a submonolayer of Ba which reacts thegermanium of the substrate 20 to form the monolayer 23 of germanidewhich, in turn, passivates the substrate surface against the subsequentreaction with oxygen.

At the outset of a process performed with the present invention, thesurface 22 of the germanium substrate 20 is cleaned to atomiccleanliness so that only germanium atoms are present at the surface 22.To this end, the surface 22 is cleaned by a process commonly referred toas a Modified RCA technique. The Modified RCA technique is a well-knownprocess involving the chemical production of an oxide at a germaniumsurface being cleaned and subsequently placing the surface in a highvacuum environment and raising the temperature of the surface to sublimethe oxide off of the surface.

The equipment used for creating a high vacuum environment about thesubstrate 20 is an ultra high vacuum (UHV) molecular beam epitaxy (MBE)facility, a fragment of which is indicated 19 in FIG. 3. The facility 19includes a container 24 having an inner chamber within which thesubstrate 20 is positioned so that its surface 22 faces downwardly, anda plurality of canisters 26 are mounted within the base of the container24 for providing a vapor source of metals desired to be added to thesubstrate surface 22 during the process of the present invention. Inthis connection, each canister 26 is adapted to hold a cruciblecontaining a desired metal and contains heating elements for vaporizingthe metal. An opening is provided in the top of each canister 26, and ashutter is associated with the canister opening for movement between aclosed condition at which the interior of the container 24 is closed andthereby isolated from the substrate surface 22 and an opened conditionat which the contents of the container 24, i.e. the metal vapor, isexposed to the substrate surface 22. In addition, an oxygen source 27 isconnected to the chamber so that by opening and closing a valveassociated with the source 27, oxygen may be delivered to or shut offfrom the chamber. The opening and closing of each canister shutter andthe oxygen source valve is accurately controlled by a computercontroller (not shown).

One other feature of the facility 19 is that a closable substrateshutter is disposed immediately below the downwardly-directed face ofthe substrate surface 20 for isolating, when desired, the substratesurface 20 from exposure to the metal vapors from the canisters 24 orthe oxygen from the oxygen source 27 while the internal pressure of thefacility chamber is raised with the oxygen from the source 27. Thesubstrate shutter is closed during one step of the present process aswill be apparent herein.

The vacuum drawn in the UHV facility 19 to complete the Modified RCAcleaning technique upon the substrate 20 is between about 10⁻⁹ and 10⁻¹⁰torr, and the substrate 20 is heated to raise the substrate surfacetemperature to a temperature sufficient to drive the oxides off of thesurface 22. In practice, such a temperature may be between about 420° C.and 500° C. for germanium, and the desired surface cleanliness may beconfirmed in-situ during the substrate heating operation by ReflectionHigh Energy Diffraction (RHEED) techniques. Briefly, a RHEED techniqueuses a high energy electron beam to diffract electrons off of thesubstrate surface 22 at a glancing angle, typically 10 kev at anincidence angle of 1 to 2 degrees. The diffraction of the electronsprovides crystallographic information while the limited penetration ofthe electron beam provides information relating to the flatness of thefilm surface. A flat surface is characterized by rods of scatteredintensity perpendicular to the film intersecting the normal Braggreflections of the crystal structure. For present purposes, thegermanium substrate 20 reaches atomic cleanliness upon the developmentof 2×1 Ge(100) at the surface 22 as evidenced by RHEED analysis.

At that point, the metal (or element) barium (Ba) is deposited upon thesubstrate surface 22 of germanium (Ge) so that a fraction, e.g. aboutone-fourth, of a monolayer of Ba covers the substrate surface 22. Inother words, the Ba metal is deposited upon the substrate surface 22until about one atom of the Ba metal overlies the germanium surface forevery four atomic sites of Ge. To this end, a vapor of the metal Ba iscreated in one of the canisters 24 (FIG. 3) and the appropriate canistershutter is opened to expose the clean substrate surface 22 to the vaporof Ba metal. As mentioned earlier, the operation of the canister shutteris controlled by a computer controller to accurately control the amountthat the Ba metal is deposited upon the surface 22. Once the exposure ofthe substrate 22 to the Ba metal is sufficient to provide the desiredfraction of the monolayer of the Ba metal, the canister shutter isclosed. This fraction of the monolayer of Ba reacts with the germaniumof the substrate surface 22 to form the desired monolayer 23 ofgermanide. The aforedescribed one-fourth monolayer of Ba is believed toresult in the optimum germanide stoichiometry. However, a fraction of amonolayer of Ba in the range of between one-sixth and one-half of amonolayer is believed to provide a germanide stoichiometry which isconsistent with the teachings of this principle.

The substrate 20 is then cooled to about 200° to 300° C. while the highvacuum environment is maintained about the substrate 20. With referenceto the phase diagram of FIG. 1, this action permits the thermodynamicpath from the germanide to the alkaline earth oxide (i.e. the compoundAO on the FIG. 1 diagram) to be completed, thereby enabling theepitaxial growth of the crystalline oxide from the germanide template.To make this change in thermodynamic direction, oxygen and the alkalineearth metal "A", in this case Ba, can be co-deposited or shuttered inappropriate proportions to remain on the stable tie line extendingbetween AGe_(x) and AO as the thin film AO grows heteroepitaxially onthe germanide.

To effect this change in the thermodynamic path going from the germanideto the oxide, the alkaline earth metal, Ba, is first deposited in theabsence of oxygen to approximately one-half monolayer and then oxygenand Ba are co-deposited to an additional one-half monolayer to providethe first full monolayer with alkaline earth oxide. Thereafter, thegrowth of the alkaline earth oxide can be continued as a pure compoundor a transition to an alkaline earth-containing perovskite oxide can bemade by alternately shuttering the alkaline earth and transition metalsources. If the purpose of the growth of the perovskite oxide is for atransistor application or other high dielectric constant capacitorapplications, e.g. DRAM (dynamic random access memory), various growthtechniques can now be employed, e.g. MBE, MOCVD, laser ablation, andsputter deposition. Additional information relative to the growth ofperovskite oxides can be found in U.S. Pat. Nos. 5,821,199 and5,830,270, the disclosures of which are incorporated herein byreference.

There is shown in FIG. 4 a STEM image of a cross section of a BaTiO₃/germanium structure illustrating the interfacial germanide constructedin accordance with the aforedescribed process applied in a molecularbeam epitaxy machine. It can be seen from this image that a perfectlycommensurate interface between the crystalline oxide and thesemiconductor is obtained.

Device considerations

Pure germanium substrates can be advantageously used in semiconductordevices wherein a crystalline oxide has been built up epitaxially uponthe germanium substrate. For example, there is shown in FIG. 5 anembodiment of a ferroelectric field effect transistor (FFET), indicated70, including a base, or substrate 72 of germanium and an overlayer 74of the perovskite BaTiO₃. During construction of the transistor 70 andprior to the deposition of the BaTiO₃ overlayer 74 upon the substrate72, a monolayer of germanide is formed at the substrate surface so thatupon completion of the transistor 70, the monolayer of germanide isinterposed between the surface of the substrate 72 and the BaTiO₃overlayer 74.

The transistor 70 is also provided with a source electrode 78, a drainelectrode 84, a gate electrode 82, and a gate dielectric 83. The BaTiO₃thin film 74 (which comprises the gate dielectric 83) is sandwichedbetween the epilayer 76 and the gate electrode 82 so as to be positionedadjacent the epilayer 76. Since ferroelectric materials possess apermanent spontaneous electric polarization (electric dipole moment percubic centimeter) that can be reversed by an electric field, theferroelectric dipoles can be switched, or flipped, and the chargedensity and channel current can be modulated. Thus, the transistor 70can be turned ON or OFF by the action of the ferroelectric polarization,and if used as a memory device, the transistor 70 can be used to readthe stored information (+or -, or "1" or "0") without ever switching orresetting (hence no fatigue).

It will be understood that numerous modifications and substitutions canbe had to the aforedescribed embodiments without departing from thespirit of the invention. For example, although the crystalline oxidelayer 31 of the FIG. 2 structure 18 is described as comprised of theperovskite oxide BaTiO₃, the crystalline oxide can be genericallyrepresented by the formula (AO)_(n) (A'BO₃)_(m) wherein "n" and "m" arenon-negative integers (i.e. any number from the set {0, 1, 2, 3, 4, . .. }), "A" and "A'" can be any element of Group IA, IIA or IVB of theperiodic table, and "B" is an element found in Group III, IVA or VA ofthe periodic table. Accordingly, the aforedescribed embodiments areintended for the purpose of illustration and not as limitation.

What is claimed is:
 1. In a process for growing a crystalline oxideepitaxially upon the surface of a Group IV semiconductor substratecomprised of germanium and wherein the process includes the steps ofdepositing an alkaline earth oxide or an alkaline earth-containingperovskite oxide upon the germanium substrate surface in alayer-by-layer build-up and wherein the alkaline earth oxide or alkalineearth-containing perovskite oxide includes an alkaline earth metal, theimprovement comprising the steps of:passivating the germanium substratesurface against the subsequent reaction with oxygen by forming amonolayer of germanide prior to the initiation of a build-up of a firstoxide layer upon the germanium substrate.
 2. The improvement of claim 1wherein the step of passivating is affected by either depositing thealkaline earth metal upon the substrate surface to a submonolayercoverage so that the alkaline earth metal reacts with the substratematerial to form a monolayer coverage of the germanideor byco-depositing the alkaline earth metal with germanium in the appropriategermanide compositions to render a deposited monolayer of the germanide.3. The improvement of claim 2 wherein the monolayer of germanide isobtained in the passivating step by reacting an amount of alkaline earthmetal at the semiconductor surface equivalent to between about one-sixthand one-half of a monolayer of the alkaline earth metal.
 4. Theimprovement of claim 3 wherein the stoichiometry of the germanidemonolayer effected by the passivating step is optimized at a coverage ofthe substrate surface of about one-fourth of a monolayer of alkalineearth metal.